In U.S. Pat. No. 4,085,644 there is described a polyphonic tone synthesizer in which a master data set is computed and stored in a main register from which it is transferred to note registers of a plurality of tone generators. The master data set defines the amplitudes of equally spaced points along a half cycle of the audio waveform of the musical tones being generated. Each tone generator receives the words in the master data set and applies them to a digital-to-analog converter at a rate determined by the fundamental pitch of the respective tones being generated by the polyphonic tone synthesizer.
One of the features of the synthesizer circuit, as described in the above-identified patent, is that the transfer of successive words from the master data set in the main register to an individual note register in the respective tone generators is synchronized with the transfer of words from the note register to the digital-to-analog converter in respective tone generators. This feature allows the master data set defining the waveform to be recomputed and loaded in the respective tone generators without interrupting the generation of the respective musical notes by the tone generators, thus permitting the waveform of a musical tone to be changed with time without interrupting the resulting musical tone.
One problem with the arrangement described in the above-identified patent is that the rate at which the waveform can be varied as a function of time is limited by the time required to transfer the data from the main register to the note registers in each of the tone generators. The transfer time in turn is limited for each tone generator by the fundamental frequency of the tone being generated. The total transfer time for each tone generator is equal to one period at the fundamental frequency of the tone being generated. If all the tone generators, e.g., 12 generators, are generating tones simultaneously in the lower registers, the total time to load the master data set in all of the tone generators can become appreciable using the arrangement described in the above-identified patent.
One solution to reducing the transfer time would be to transfer the entire master data set into the note register of a tone generator in a period of time which was less than the time required to transfer successive digital values from the note register to the digital-to-analog converter of the tone generator. This would permit the master data set in the note register to be updated without interrupting the flow of data to the digital-to-analog converter and hence without interrupting the resulting audio signal at the output of the tone generator. However, to generate audio waveforms having 32 harmonics, a minimum of 64 data points is required for the master data set in the note register to define one cycle having the desired waveshape. To reproduce a musical note for the highest note on the musical instrument, which preferably is the note C.sub.7, corresponding to a fundamental frequency of 2093 hz. would require a transfer frequency of at least 8.57 mhz., i.e., 2093.times.64.times.64. This is too high a transfer frequency for low-cost microelectronic devices preferably used in electronic musical instruments.